Several clinically unrelated neurodegenerative disorders, termed conformational diseases, are character- ized by a common pathophysiology that involves aggregation and accumulation of misfolded proteins. All of these diseases activate cell stress signaling pathways, mainly the endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR) pathway. In particular, the proximal UPR kinase, PERK, is abnormally active in Alzheimer's disease (AD), Progressive Supranuclear Palsy (PSP) and Frontal Temporal Dementia (FTD)- affected brain regions. Encouragingly, pharmacological inhibition of PERK activity appears to restore neuro- logical deficits associated with these disorders in preclinical mouse models, including loss of cognition and memory. However, current PERK inhibitors are toxic, necessitating the need to define the precise mecha- nisms that regulate PERK activity in these degenerative processes. We find that in addition to luminal regula- tion, PERK activity is enhanced by an increase in cytosolic Ca2+. Surprisingly, our super resolution microscopy measurements indicate the majority of PERK molecules may require only transient dimerization for activation. Our central hypothesis is that dysregulation of PERK activity can lead to neurodegeneration. In Aim 1, we will test the hypothesis that PERK is activated by the transient formation of dimers, while PSP-associated PERK coding variants suppress PERK inactivation. In Aim 2, we will test the hypothesis that leakage of ER Ca2+ leading to PERK-calcineurin binding enhances PERK autophosphorylation by stabilizing dimer formation. Successful completion of Aims 1 and 2 will precisely determine luminal and cytosolic regulation of PERK under normal and pathologic conditions. In Aim 3, we will test the hypothesis that PERK dimerization in human brain cells and tissue from AD patients is increased by ER stress, soluble mutant tau oligomers and/or increases in cytosolic Ca2+. We will also measure PERK activation in a neurodegenerative mouse model. Successful com- pletion of Aim 3 will validate our in vitro data in neurodegenerative pathophysiology in vivo. Super resolution microscopy, molecular, biochemical and mouse genetic approaches will be used to fulfil these aims. Data generated from this proposal will potentially impact all conformational diseases and identify targets for rapid and integrated therapy.